Although platforms, bins, containers, man-cages, and other such appliances can be elevated or transported using a variety of means, such as cranes and the like, a convenient way to elevate or transport such appliances is by employing a forklift vehicle. Forklifts as generally known in the art are self-propelled vehicles having a lifting member (e.g., a lifting "fork") usually located on the front end and which is used to lift goods-loaded pallets and other heavy or bulky items for transportation to remote locations or for movement from one elevation to another. A typical lifting member is a lifting fork having two outwardly projecting parallel tines. Such a lifting fork is typically mounted on a substantially vertical track, or "mast" on the vehicle The mast is equipped with a winch or analogous mechanism used to raise and lower the lifting member relative to the vehicle.
The term "implement" as used herein encompasses various types of industrial load-carrying appliances such as platforms, bins, containers, man-cages, and the like, that are adapted to be engaged with a lifting member on a forklift vehicle so as to permit personnel, material, and equipment loaded on or in such appliances to be elevated by the forklift vehicle to high, otherwise inaccessible locations, or to be transported by the forklift vehicle from one location to another.
To elevate an implement using a forklift equipped with a lifting fork, the tines of the fork are typically placed beneath the implement. Some implements are provided with underside grooves, channels, or sockets to ensure that the tines are placed properly relative to the mass of the implement and to prevent the implement from sliding laterally off the tines.
Many forklifts are equipped with means for adjusting the forward and rearward tilt of the mast which, in turn, adjusts the tilt of the tines from horizontal. Adjusting the fork so that the tines are angled downward relative to horizontal can pose a substantial hazard in that the implement can slide in a forward direction off the fork. Even if the tines are not angled downward, it is possible for an implement to slip forward off the fork if a forward-moving forklift vehicle carrying the implement stops suddenly, or if weight of the load in or on the implement shifts position on the tines. Sliding of the implement on the tines in a rearward direction does not pose as great a risk since such movement tends to place the implement more completely on the tines. Also, excessive rearward movement of the implement on the tines is usually obstructed by the mast and by various abutting plates or bars situated behind and above the tines.
Several methods and apparatus are known in the art for securing objects to the tines of a lifting fork. For example, U.S. Pat. No. 5,096,018 to Dickinson, Jr. discloses a clamp adapted to be mounted to a tine socket on an implement. The clamp applies a gripping force to the underside of the tine whenever the fork has elevated the implement off a reference surface, and releases the tine whenever the implement is resting upright on the reference surface. The clamp comprises a lever pivotably mounted at about mid-length to the tine socket, at least one gripping cam rotatably mounted to one end of the lever, and a cam-release member mounted to the opposing end of the lever. The lever is biased using strong springs to maintain the gripping surface of the cam in contact with the underside of the tine whenever the tine is in the socket and the implement is elevated. The cam has an outwardly spiraled profile and is allowed a limited degree of rotational freedom about its axis. Whenever the cam is engaged against the underside of the tine, rotation of the cam causes the gripping force applied by the cam to increase principally because, as the cam rotates, the springs stretch to a longer length so as to apply more tension to the lever; and the effective spring lever arm changes to apply more torque to the lever. The cam-release member contacts the reference surface whenever the implement is resting upright thereon. This causes the lever to pivot against the bias and draw the cam away from contact with the underside of the tine, thereby allowing the tine to be withdrawn from the socket.
The primary disadvantage of a clamp according to Dickinson, Jr. arises from the location of the cam on an end of the lever that does not serve as the fulcrum of the lever. Thus, as the tine is being urged out of the socket, the resulting rotation of the cam causes the cam pivot axis to move away from the tine. That is, the cam pivot axis is not fixed. The springs connected to the lever cannot practicably be made strong enough to prevent such movement of the cam pivot axis. Consequently, such a clamp may not be capable of gripping a tine strongly enough in all instances to prevent an implement from sliding off the fork.
Other disadvantages of a clamp according to Dickenson Jr. are the following: First, debris can accumulate atop the shoe employed for releasing the cam. Such an accumulation can prevent the lever from pivoting sufficiently to release the cam. Second, the shoe depending from the lever is easily caught on obstacles which can cause substantial damage to the shoe and other portions of the clamp mechanism. Third, the clamp relies substantially upon the teeth in the cam to grip the underside of the tine; if the teeth should become worn, then the gripping power of the cam is significantly reduced.
U.S. Pat. No. 3,889,833 to Thomas discloses plural manually pivotable "square Z" latches provided on a manbasket for engaging the abutting plate of a forklift. A disadvantage of such latches is that they are usable only with a forklift having an abutting plate with the proper depth and located the proper distance above the tines. Also, such latches are biased by gravity to return to the latched position, which is not fail-safe. For example, if the latch journal fails to allow free rotation of the latch due to rust or incursion of dirt, the latch may not engage the abutting plate, particularly if one forgets to manually engage the latch. Another disadvantage is that the latches must be manually opened, which can be inconvenient.
U.S. Pat. No. 3,101,128 to Dane discloses a personnel platform provided with sets of parallel channels adapted for receiving the tines of a lifting fork therebetween. Each set of channels has an opening into which a tine is inserted. Each opening is partially obstructed with a vertical plate adapted to become situated behind the heel of the tine whenever the manbasket is lifted off the ground by the lifting fork. Unfortunately, providing such a feature requires that the personnel platform rest in a tilted orientation on the ground to allow insertion of the tines. Also, the platform is provided with an inwardly tilting side panel to permit incursion of the forklift mast between two lateral sides of the platform. Hence, Dane discloses an elaborate mechanical interconnection between the tilting side panel and a pair of swingable legs which keep the platform in a tilted position on the ground. The fact that the platform must remain tilted on the ground is disadvantageous because workers are discomforted thereby. Also, the tilted floor can make it difficult to stabilize equipment and tools placed on the platform until the platform is elevated by the forklift. Also, proper placement of the platform on the tines requires appreciable manual intervention, including moving the tilted side panel into a vertical position after the platform has been lifted off the ground.
Another means known in the art for securing an implement to a lifting member includes a chain passed around the mast and fastened to the implement. A disadvantage of this method is that it is easy to forget or ignore fastening the chain.
Another means known in the art is to fasten an implement to a lifting member using pins or screws or the like. This method has the disadvantage in that pins or screws must be manually engaged against the lifting member before elevating the implement and manually released when the implement is not in use. Also, screws are vulnerable to damage by the lifting member. For example, U.S. Pat. No. 4,049,146 to Decker discloses a screw mechanism which is used to secure an implement to the tines of a lifting fork.
Hence, there is a need for an apparatus for securing an implement to a lifting member which will reliably prevent the implement from slipping forwardly on the lifting member when the implement is elevated by the lifting member.
There is also a need for such an apparatus wherein the securing of the implement to the lifting member is automatic (i.e., requiring no deliberate action by personnel to engage the securing means before the implement is elevated).
There is also a need for such an apparatus that automatically disengages the lifting member from the implement whenever the implement is resting upright on the ground or other reference surface.
There is also a need for such an apparatus that can be used to secure an implement to lifting members of different makes and models of forklift vehicles without the need for intervening adaptive action.
There is also a need for such an apparatus that is of a simple design utilizing a minimum of mechanical parts.